1. Field of the Invention
The present invention relates to an adaptive equalizer, and more particularly to a linear equalizer and a decision feedback equalizer adapted for removing intersymbol interference in a digital data transmission system.
2. Description of the Prior Art
Equalizers of this type are disclosed in U.S. Pat. No. 4,852,090 to Borth, for example. FIG. 3 is a block diagram showing the structure of such an equalizer. FIG. 4 shows the format of a transmission data frame employed with this equalizer. In FIG. 4, the symbols L1, M1, L2 and M2 represent a first training field, a first user's data field, a second training field and a second user's data field, respectively. Predetermined data are provided in the first and second training fields, and the same data have been stored in a memory unit 109 shown in FIG. 3.
Next, the operation of the equalizer mentioned above will be described. A signal y(n) received by a first input terminal 101 is applied to a shift register 102 and a coefficients regeneration unit 107. The signals stored in the shift registers 102 are multiplied in multiplication blocks 103 by associated coefficients C.sub.i (i=-N . . . M) output from the coefficients regeneration unit 107, and the products are in turn applied to an adder 104, wherein both N and M are integers larger than zero. The adder 104 calculates the sum z(L) of all the products output from the multipliers 103 to produce a summation result, which is in turn applied to a decision block 105. The sum z(L) may be defined by the expression: ##EQU1##
The decision block 105 outputs a decision result x(L) to an output terminal 110. A switch 108 has input terminals 108a and 108b. During first and second training intervals, switch 108 sequentially supplies the data stored in the memory unit 109 to an inverting input (-) of an adder 106, which substantially acts as a subtracter. During first and second user's data intervals the switch 108 sequentially supplies the output x(L) of the decision block 105 to the inverting input of the subtracter 106.
More specifically, the desired signals in the equalizer are, during the training intervals, the data stored in the memory unit 109, and alternatively, during the user's data intervals, the decision result output from the decision block 105. The subtracter 106 subtracts the output of the switch 108 from the output z(L) of the adder 104, and outputs the result of the subtraction to the coefficients regeneration unit 107 in the form of an equalization error signal e(L). In the coefficients regeneration unit 107, the received signal y(n) and the equalization error signal e(L) are used to update the coefficients of the multipliers 103 so as to follow up variations in the characteristics of the transmission line or transmission channel. The thus regenerated coefficients are supplied to the multipliers 103. Algorithms for coefficients regeneration are known, and include the RLS (Recursive Least Square) algorithm, the LMS (Least Mean Square) algorithm, and the like. FIG. 5 shows a block diagram of the coefficients regeneration unit 107 in the case of using the RLS algorithm for the purpose of coefficients regeneration. Now, the following equations are given: EQU q.sup.T (L)=(y(L+N), y(L+N-1), . . . , y(L), . . . , y(L-M)) EQU c.sup.T (L)=(c.sub.-N (L), c.sub.-N+1 (L), . . . , c.sub.0 (L), . . . , c.sub.M (L))
where T represents transposition of a vector, and the underline represents a column vector. Further, in the block diagram shown in FIG. 5, k(L) is the (N+M+1)-th order vector, and P(L) is the (N+M+1)-th order square matrix.
First, upon receipt of a start signal supplied through a second input terminal 111, P(L) and c(L) are set to initial values P(0) and c(0), respectively. Thereafter, arithmetic operations are performed, whenever the received signal y(L+N) is entered through the first input terminal 101, in accordance with the expressions: EQU k(L)=p(L-1)q(L){1+q.sup.T (L)p(L-1)q(L)}.sup.-1 EQU c(L)=c(L-1)+k(L)e(L) EQU p(L)=p(L-1)-k(L)q.sup.T (L)p(L-1)
where c.sub.i (L) (i=-N . . . M) is output to the multipliers 103.
The arithmetic operations in the coefficients regeneration unit 107 are performed usually by program sequences in a DSP (Digital Signal Processor).
However, according to the conventional equalizers of the type stated above, if rapid variations in the transmission channel characteristics occur, regeneration of tap coefficients of the equalizer does not follow the variations in the transmission channel characteristics. Thus, the tap coefficients at the time of termination of the first user's data field become independent of the transmission channel characteristics. Consequently, the conventional equalizers have the drawback that proper tap coefficients cannot be obtained in the second training interval indicated by the symbol L2, even if proper desired signals can be obtained in the second training field.